The hydraulic fracturing technology has been widely used in the development of low-permeability oil and gas fields as a major measure for increasing production of oil and gas wells and augmented injection of water wells, and has made important contributions to stable production of oil and gas fields. The hydraulic fracturing process refers to pumping high-viscosity pre-flush fluid into a target reservoir, forming fractures at high pressure and extending the fractures, and then pumping sand-carrying fluid mixed with a proppant, wherein the sand-carrying fluid can continue to extend the fractures, and at the same time carry the proppant deep into the fractures; and finally, enabling the fracturing fluid to break and degrade into low-viscosity fluid which then flow to the bottom of the well and later flows back, thereby leaving a flow channel with high conductivity formed by the proppant supporting the fracture wall in the formation to facilitate flowing of oil and gas from the far-field formation to the bottom of the well.
Since the 1st hydraulic fracturing in the United States in 1947, after more than 60 years of development, the hydraulic fracturing technology has achieved an amazing development from theoretical research to field practice. For example, a fracture extending model has developed from two-dimension to pseudo-three-dimension and full three-dimension; a dynamic prediction model of a fractured well has developed from an electric simulation chart and a steady state flow model to a three-dimensional and three-phase unsteady model; the fracturing fluid has developed from crude oil and clean water to low-, medium-, and high-temperature series of high-quality, low-damage, delayed-crosslinking guar gum organoboron “double-variable” fracturing fluid systems and clean fracturing fluid systems; a proppant has developed from natural quartz sand to medium- and high-strength man-made ceramsites; fracturing equipment has developed from a low-power cement truck to a 1000, 2000, or 2500 fracturing truck; the single-well fracturing construction has developed from small-scale, low sand-to-liquid ratio to ultra-large, high-sand-fluid ratio fracturing operations; the field of fracturing applications has developed from specific low-permeability oil and gas reservoirs to simultaneous development of ultra-low-permeability and medium- and high-permeability oil and gas reservoirs (and sometimes sand-control fracturing).
However, from the point of view of the hydraulic fracturing technology and its development, all fracturing technologies are currently based on the process of forming fractures with a fracturing fluid, then injecting a solid proppant to hydraulic fractures, and supporting the fractures to keep them open, thereby obtaining a flow channel with high conductivity.
In 2010, Schlumberger proposed that the fracture conductivity in a HIWAY high-speed channel was not affected by the permeability of a proppant, and oil and gas do not pass through a proppant pack but instead flows through a high-conductivity channel. However, its implementation requires strict requirements on a perforating process, a pump injection process, and pump injection equipment. The construction cost is high and the process is complicated. It is also necessary to inject a proppant into the formation to open fractures.
The conventional guar gum fracturing fluid system and sand fracturing generally have the following problems:
(1) if the fracturing fluid is not thoroughly broken and returned, it will seriously injure the conductivity of artificial fractures and reduce the matrix permeability near the fractures;
(2) with respect to high temperature deep wells, in order to maintain the sand carrying capacity of the fracturing fluid at high temperature, the concentration of additives such as guar gum and crosslinker is increased, resulting in further increase of the residue content, further increase of frictional resistance, and further occurrence of the problems, such as gum breakage and flowback;
(3) for sand fracturing, in order to pursue high conductivity, sand is introduced at a high sand ratio, which can easily lead to sand plugging and other accidents;
(4) with the increase of production time after construction, problems such as embedding, deformation, and backflow of conventional proppants such as ceramsite and quartz sand will result in a significant decrease in post-pressure conductivity, and the construction validity will be greatly shortened.
The above problems often lead to a significant reduction in fracture conductivity. Therefore, the fracture permeability measured after post-pressure well testing can only reach one-tenth or even one-hundredth of the permeability in a laboratory.
In order to effectively improve the post-pressure permeability, reduce the reservoir damage caused by the fracturing fluid, and improve the overall fracturing effect, the present invention proposes a phase-change hydraulic fracturing process in which a phase-change fracturing fluid forms a solid support by means of its own phase-change, and a non-phase-change fracturing fluid injected together with the phase-change fracturing fluid flows back after the fracturing is completed to keep out of the space within the fracture to form an oil and gas seepage flow, such that the fracture permeability is greatly improved.